According to the most popular theory, the Moon was created when a Mars-sized planet struck the Earth. The impactor was vaporised, and Earth’s mantle was blown off into Space. The pieces of mantle were large enough to coalesce in Earth orbit until they formed the Moon. The Giant Impactor theory became popular after Apollo Moon samples were found to resemble parts of Earth’s mantle.
The Moon first coalesced less 1/4 its present distance from Earth. Since that time 4.5 billion years ago the Moon has been slowly drifting away. This is interpreted as tidal forces transferring angular momentum from Earth’s rotation to the Moon. Apollo’s Lunar Laser Ranging Experiment measured this distance increasing at 3.82 cm/yr, anomalously high. If the Moon were today gaining angular momentum at that rate, it would have been in the same place as Earth only 1.5 billion years ago.
If the speed of light were slowly decreasing, time for light to return from the Moon would increase each year, making the Moon appear to recede faster as seen by LLRE. Change in the speed of light, predicted by the simple expression GM=tc^3, precisely accounts for the lunar anomaly. This is striking evidence that the speed of light is slowing today.